1. Field of the Invention
The present invention relates to an optical transmission system, and more particularly, to an optical transmission system for compensating for transmission loss, an optical signal transmitting/receiving apparatus of the optical transmission system for compensating for transmission loss, and a parallel/serial data converter for transmitting high speed data in the transmitting apparatus.
2. Description of the Related Art
An optical transmission system includes an optical signal transmitting apparatus, an optical fiber, and an optical signal receiving apparatus. The transmitting apparatus converts an electrical signal to be transmitted into an optical signal, using a laser diode. The optical fiber transmits the optical signal converted by the transmitting apparatus to a receiving apparatus. The receiving apparatus receives the optical signal transmitted through the optical fiber at a photo diode and converts the received signal into an electrical signal.
The transmitting apparatus provides driving current to the laser diode in order to output the optical signal using the laser diode. Namely, the transmitting apparatus controls the driving current and controls the optical output power of the laser diode. Conventional transmitting apparatus commonly control the amount of driving current by controlling an external resistance value.
FIG. 1 is a schematic block diagram showing a conventional single channel optical signal transmission system. Referring to FIG. 1, a transmitting apparatus 10 includes an optical driver 12, a bias and modulation current controller 14, a bias resistor Rbias, a modulation resistor Rmod, and a laser diode 16. A receiving apparatus 20 includes an optical receiver 26 and a photo diode 24. Here, the bias resistor Rbias and the modulation resistor Rmod can exist outside the transmitting apparatus 10.
In FIG. 1, the optical driver 12 of the transmitting apparatus 10 converts externally-received input data DIN into a current signal. The optical driver 12 further changes the magnitude of the converted current signal in response to the bias current Ibias and the modulation current Imod, which are provided by the bias and modulation current controller 14. The magnitudes of the bias current Ibias and the modulation current Imod can be controlled by changing the magnitudes of the bias resistor Rbias and the modulation resistor Rmod. Also, the optical driver 12 generates a magnitude-variant current signal as the driving current for driving the laser diode 16. The laser diode 16 generates an optical signal having an optical output power corresponding to the driving current. The optical signal generated by the laser diode 16 is transmitted to the receiving apparatus 20 through an optical fiber 18. The photo diode 24 of the receiving apparatus 20 receives the optical signal transmitted from the optical fiber 18 and converts the received optical signal into a current signal. The optical receiver 26 recovers original data from the current signal converted by the photo diode 24. In FIG. 1, DOUT represents recovered data.
The conventional optical transmission system shown in FIG. 1 controls the amount of driving current Id output from the optical driver 12 by changing the magnitudes of the bias resistor Rbias and the modulation resistor Rmod, thus controlling the magnitudes of the bias current Ibias and the modulation current Imod. Namely, the optical output power of the laser diode 16 is controlled by changing the resistances of the bias resistor Rbias and the modulation resistor Rmod.
The conventional optical transmission system shown in FIG. 1 is an open loop system, in which the optical output power of the laser diode 16 can be controlled only at the transmission end.
For example, when transmission loss is generated while the optical signal is transmitted through the optical fiber, or received by the receiving apparatus, errors can be generated in the data recovered by the optical receiver of the receiving apparatus. Therefore, the transmitting apparatus transmits an optical signal having appropriate optical output power so that the optical transmission system can maintain a predetermined transmission efficiency, taking into account transmission loss. The transmission efficiency can change according to the characteristics of the devices used for the optical transmission system such as the optical diodes for transmission, the optical fibers, and the optical diodes for reception. Also, the transmission efficiency of the optical transmission system can change as the length of the optical fiber changes. Also, the transmission efficiency of the optical transmission system can change since the characteristics of the devices used as the optical fiber may not optimally match. Transmission loss can be different from what was expected due to the change in the transmission efficiency. Accordingly, errors can be generated in the data recovered by the optical receiver of the receiving apparatus.
Also, in the case of a multi-channel optical transmission system for transmitting multimedia data such as graphic data, uniformities of the laser diode, the optical fiber and the photo diode can vary in each channel. Therefore, in the case of the multi-channel optical transmission system, transmission efficiency varies in each channel. Accordingly, errors can be generated when data is transmitted.
In the conventional open-loop optical transmission system, the optical output power is controlled according to the characteristics of the transmitting apparatus. However, the change in transmission efficiency according to the characteristics of the optical fiber and the receiving apparatus is not taken into account. Namely, the transmission efficiency changes due to the characteristics of the optical fiber and the receiving apparatus. Accordingly, it is not possible to compensate for errors generated when data is transmitted.
Also, in order to minimize the change in transmission efficiency caused by the characteristics of the optical fiber and the receiving apparatus, the error allowance range of the standards of the devices used for the optical transmission system must be small. Therefore, when the conventional optical transmission system of the open loop is constituted, restrictions are placed on selection of the devices. Accordingly, it is expensive to constitute the optical transmission system.